Jib crane



F. N. BECKER Dec. 19, 1967 JIB CRANE Filed Feb. 2, 1966- 2 Sheets-Sheet l INVENTOR: Fxzxmcus N. Beam-s2 ATTORNEYS Dec. 19, 1967 F. N. BECKER 3,358,849

JIB CRANE Filed Feb. 2, 1966 2 Sheets-Sheet 2 o INVENTOR.

FRAN as N. BECK E-R F BYWMJ M4M ATTORNEYS United States Patent 3,358,849 33 1B CRANE Francis N. Becker, Charlotte, N.C., assignor to Parks- Cramer Company, Fitchburg, Mass, a corporation of Massachusetts Filed Feb. 2, 1966, Ser. No. 524,422 6 Claims. (Cl. 21263) ABSTRACT OF THE DISCLOSURE A ji-b crane having a base having a vertically extending bore and rotatable mast supported in the bore with the surfaces defining the bore and the adjacent exterior surfaces of the mast being unmachined, loose ball members disposed between and in direct rolling contact with the unmachined surfaces of the bottom of the bore and the lower end of the mast, and ball members arranged in vertically spaced apart toroidal rows between and in rolling contact with the unmachined surfaces of the sides of the bore and the mast.

My invention relates to a jib crane and, more particularly, to a jib crane which is less expensive to manufacture than conventional jib cranes, more easily adapted to varying ranges of load capacity, and freely rotatable under load.

In a conventional jib crane, a supported end of a horizontal cantilever boom is secured to a crane head or box assembly, which in turn is rotatably supported upon the upper extremity of a stationary vertical mast. Inasmuch as an object being lifted by a lifting device, such as a hoist, is located at some position along the length of the boom, the resulting load imposes :a vertically downward thrust force and an overturning moment, both of which must be transferred from the rotatable boom to the stationary mast through bearings provided between the box assembly and the mast. The overturning moment may be resolved into two overturning forces of equal magnitude, acting in opposing directions along parallel and spaced apart lines. The thrust force and a first one of the overturning forces are transmitted through a thrust and radial bearing mounted at the upper extremity of the stationary mast, while the other of the overturning forces is transmitted through rollers carried in the box assembly at a position spaced downwardly from the thrust and radial bearing and engaging the exterior of the mast. The use of a box assembly to rotatably mount the boom on the stationary mast not only increases the weight of the structural members which must be supported above the ground, but raises difficulties in assembling the crane for use at the time of installation.

In order to decrease the force required to rotate the boom of a conventional jib crane, the thrust and radial bearing is usually a high precision and relatively expensive anti-friction bearing, which requires precision machining of surfaces of the crane head and the upper extremity of the mast which engage and retain the bearing. Further, special bearing surfaces must be provided on the mast for the rollers, and the rollers must be made adjustable to track on the surface of the stationary mast with the greatest possible contact area and the lowest possible fricion to obtain the least possible wear of the mast and rollers. It may readily be understood that manufacture of such machined and precision elements for a, jib crane involves considerable expense.

Under the dead load imposed by the weight of the structural members of a conventional jib crane, the members deflect so as to position the boom at some downwardly projecting angle from a true horizontal position.

3,358,849 Patented Dec. 19, 1967 Care must be used in the initial installation of a jib crane in which a boom and box assembly are rotatably supported on a stationary vertical mast in order to assure that the vertical mast is plumb, or truly vertical. The deflection of the structural members has, in the past, been partially overcome by shims inserted between the boom and the box assembly to raise the boom a distance comparable to the deflection expected under dead load and thereby position the boom in an approximately true horizontal position. However, the deflection of the box assembly and mast is not predictable for all positions of rotation of the boom and box assembly about the central vertical axis of the stationary mast, and the rollers may not properly track on the surface of the mast. As a result, while the boom is shimmed to approximate a true horizontal position at one predetermined location of rotation, rotation of the boom to another such location of rotation, rotation of the boom to another such location, as during positioning of the boom to overlie an object to be lifted, may result in deflection of the boom from the approximate horizontal position. Upon the occurrence of this condition, which may be further complicated by displacement of the stationary m-ast from a true vertical, or plumb, position, resultant forces will arise and act on the lifting device, to move the same along the boom, or act on the boom and box assembly to rotate the same, rather than permitting the crane operator to retain control over such movements as would be desirable.

The deflection effects of dead load upon the structural members of a jib crane, and the difficulty of adapting a box assembly and mast to varying load capacity ranges, have resulted in a requirement that a jib crane manufacturer design and build cranes for each rated load capacity with little interchangeability of parts, as the elements cannot readily be adapted to varying load capacity ranges. This low interchangeability between jib cranes of varying load capacity ranges results in greater expense in providing cranes for each specific load capacity.

It is an object of this invention to provide a jib crane in which the aforementioned difliculties and deficiencies of conventional ji-b cranes are avoided. This object is realized in a jib crane which, in accordance with this invention, does not require high precision machine work in construction and is readily adapted to varying ranges of load capacity. The jib crane of this invention is characterized by having a base element and a mast element, with at least a portion of one of the elements being rotatably received within the other so that the mast element is rotatably supported relative to the base element by inexpensive bearing means interposed between the base and mast elements. Preferably, the mast element is supported in a bore in the base element and the bearing means are positioned between opposed thrust bearing surfaces on the mast and base elements to form a thrust bearing means, for supporting the mast against thrust forces, and encircling the m-ast element at vertically spaced apart areas to form radial bearing means, for supporting the mast against overturning forces.

It is a more specific object of this invention to provide a jib crane of the type described in which a mast is rotatably mounted within a vertically extending bore of a base and has a horizontal boom secured at one end thereof to the mast for rotation therewith. Thrust bearing means in the form of uniformly sized loose ball members are interposed between the lower extremity of the mast and the base, and radial bearing means in the form of uniformly sized loose ball members are interposed between the mast and the bore at vertically spaced apart areas.

Some of the objects and advantages of this invention having been stated, others will appear as the description ing drawings, in which FIGURE 1 is a perspective view of the jib crane in accordance with this invention;

FIGURE 2 is a section view, taken along the line 22 in FIGURE 1, of the jib crane of this invention;

FIGURE 3 is a section view, taken along the line 3-3 in FIGURE 2, of a portion of the jib crane of this invention;

FIGURE 4 is a section view, taken along the line 4-4 of FIGURE 2, of a portion of the jib crane of this invention;

FIGURE 5 is a perspective view, on enlarged scale, of a portion of the jib crane of FIGURE 1, taken as indicated by the arrow 5 in FIGURE 1;

FIGURE 6 is a view similar to FIGURE 2 of a modified form of the jib crane of FIGURE 1;

FIGURE 7 is an elevation view, in partial section, of a second modified form of the jib crane of FIGURE 1, as adapted for varying load capacity ranges; and

FIGURE 8 is a section view, taken as indicated by the line 88 in FIGURE 7, of the jib crane of FIGURE 7.

Referring now more particularly to the drawings, a jib crane in accordance with my invention is there shown and indicated generally at 10 in FIGURE 1. The crane comprises a base element, indicated generally at 11, a vertical mast element 16 rotatably received within the base element 11, and a horizontally extending boom 26 having one end thereof secured to the mast and being rotatable with the mast. Preferably, the boom 26 is a conventional girder cross-section, upon which a lifting device 29 such as a hoist may be supported for translational movement along the boom.

In the jib crane 10, the base 11 is constructed of three constituent members, to have a vertically extending bore with an interior cylindrical surface therein. In particular, the base 11 comprises a base plate 12, of suitable steel plate, a hollow cylindrical cage member 14 mounted perpendicular to the base plate 12 and secured thereto as by welding, and a conical reinforcing member 15. The cage member suitably may be a section of steel pipe or tubing, of proper internal diameter to define a bore suitable for receiving the mast 16 and the bearing means, as described hereinafter.

The reinforcing member preferably is a rolled conical form of steel plate tapering from an upper internal diameter equal to the external diameter of the cage member 14 to a suitable larger lower diameter. The upper extremity of the conical reinforcing member 15 is secured to the exterior of the cage member 14, as by welding, and the lower extremity is similarly secured to the base plate 12. Through the use of conical reinforcing member 15, the deflection of the base 11 as the mast 16 and boom 26 are rotated to various locations is maintained substantially constant, with beneficial results in permitting adaptation of the basic jib crane structure to a range of various load capabilities, as described in greater detail hereinafter. Further, the amount of welding required in fabrication of the base 11 is substantially reduced, as compared to conventional constructions.

Preferably, the mast 16 is a section, of suitable length, of a hollow cylindrical member such as steel pipe, as illustrated. However, other suitable sections, such as conventional girder cross-sections, may be chosen for the mast 16, provided that such an alternate section is provided with an exterior cylindrical surface which may be radially spaced apart from the interior cylindrical surface of the bore in the base 11, as defined by the cage member 14. The radially spaced apart interior and exterior cylindrical surfaces of the mast and base define therebetweena hollow right cylindrical volume, within which the radial bearing means of the crane of my invention are accommodated, as will be hereinafter described.

In order to support the mast 16 and boom 26 of the jib crane 10 under the thrust and overturning moment forces arising when a load is imposed on the crane, thrust bearing means are provided and interposed between the base 11 and the mast 16 for supporting the mast 16 and boom 26 against vertically downwardly directed forces, and radial bearing means are provided to support the mast 16 against overturning moment forces. More particularly, the lower extremity of the mast 16 has a thrust bearing surface member 18 secured thereto, as by welding. The thrust bearing surface member 18 does not require high precision machining, and suitably may be a circular member flame cut from steel plate to have a diameter greater than that of the section chosen to form the mast 16 and slightly less than the internal diameter of the bore defined by the cage member 14 of the base 11.

Interposed between the upper horizontally extending thrust bearing surface, provided by the member 18 adjacent the lower extremity of the mast 16, and the lower horizontally extending thrust bearing surface, provided by the base plate 12 of the base 11 and underlying member 18, are a number of loose ball members 20. The balls 20 are of uniform size and any suitable diameter available commercially or readily manufactured, and are disposed in a single layer preferably without excessive unfilled space, to retain substantial freedom of movement. During the construction of the jib crane 10 of this invention, the balls 20 for the thrust bearing means may be positioned simply by pouring them into the bore defined by the cage member 14 until the available area on the base plate 12 and within the bore is uniformly filled with a single layer. Inasmuch as precision machining is not required for the upper surface of the base plate 12, defining the lower horizontally extending thrust bearing surface, and the lower surface of the thrust bearing member 18, defining the upper horizontally extending thrust bearing surface overlying the surface of the base plate 12, it may be under stood that the thrust bearing means is inexpensively provided. Due to the provision of separate radial bearing means to accommodate overturning forces, the thrust bearing means need accommodate only the downward thrust force on the mast and boom. As the balls 20 retain freedom of movement, and a large number of such members are provided, an adequate bearing surface is provided and low friction is obtained through rolling movement of the balls.

In order to support the mast 16 and boom 26 against overturning forces, the crane 1t? of my invention is provided with radial bearing means positioned at vertically spaced apart areas to encircle the mast 16 and be interposed between the radially spaced apart internal and external cylindrical surfaces provided by mast element 16 and the cylindrical bore defined by the cage member 14 of the base element 11, as heretofore described. The radial bearing means includes bearing elements adjacent the lower extremity of the mast 16 in order toaccommodate one of the overturning forces, and bearing ele ments adjacent the upper extremity of the cage member 14 to accommodate the other of the overturning forces.

Adjacent the lower extremity of the mast 16, the outer periphery of the thrust bearing member 18 preferably extends outwardly beyond the outside diameter of the section forming the mast 16 at a distance sufiicient to support a toroidal row of a plurality of uniformly sized loosely disposed ball members 2, similar to the balls 20 heretofore described. A plurality of toroidal rows of bails 22 are preferably provided, with immediately adjacent rows being separated by separator rings or members 21, loosely encircling the mast 16. The separator rings 21 need not require high precision machining, and suitably may be annular members flame cut from suitable steel plate. The separator rings 21 are not secured to the mast 16, but are loosely interposed between adjacent rows of balls 22. A sufiicient number of balls 22 are included in each toroidal row to encircle the mast 16 without sufiicient remaining space to permit the insertion of an additional ball. The diameter of the balls 22 is slightly less than one-half the difference between the diameters of the ex terior cylindrical surface of the mast 16 and the interior cylindrical surface of the bore defined by the cage member 14 of the base 11, with sufiicient looseness to permit ease in assembly and free operation of the assembly. A sufficient number of toroidal rows of balls 22 are provided to form a bearing contact area between the cage member 14 and the mast 16 adequate to accommodate the overturning force. Immediately above the uppermost toroidal row of ball members 22 is positioned a retaining ring or member 24, substantially similar to the separator rings 21, which is welded to the mast 16, to restrain the rows of balls 22 against vertical movement and retain the balls 22 in the desired toroidal rows.

Adjacent the upper extremity of the cage member 14 is provided a radial bearing construction substantially similar to that described immediately heretofore, with the lower vertical extremity of the bearing construction being defined by a retaining ring or annular member 24, which supports the lowermost toroidal row of balls 22. Adjacent toroidal rows of balls 22 are separated by separator rings or members 21, as described heretofore, and a retaining ring or member 24 is provided at the upper extremity of the bearing construction, immediately adjacent the upper extremity of the cylindrical bore defined by the cage member 14 of the base 11.

In order to secure the boom 26 to the mast 16 for rotation therewith and in a cantilever configuration, the upper extremity of the mast 16 is preferably provided with a flat plate mounting member 25 (FIGURE to which the lower flange of the girder form used as the boom 26 may be secured by suitable means, such as by bolting. Due to the absence of relative rotation between the boom 26 and the mast 16 in the jib crane of my invention, the manner of securing the boom 26 to the mast 16 may be varied in order to obtain a boom support configuration other than cantilever, should such be desired. When the boom 26 is to be used in cantilever configuration, and the mast 16 is provided with mounting member 25, both thrust and bending moment forces are transmitted from the boom 26 to the mast 16 through the right angle cantilever support connection provided by the mounting member 25. Suitable shims may be inserted between the mounting member 25 and the lower flange of the girder form used as the boom 26 in order to correct the dead load deflection of the boom and mast, by raising the free end of the boom to position the boom in a substantially horizontal position. Where a lifting device 29, such as a hoist, is mounted on the boom 26 to move therealong, the boom additionally may have stop members 28 secured thereto, to limit the distance of travel of the lifting device 29.

Due to the rotation of the mast 16 with the boom 26 in the jib crane in accordance with my invention, the dimculties and deficiencies resulting from bearings interposed between a stationary vertical mast and a box assembly and the increased weight of the structural members required for such a conventional jib crane are avoided. Further, the dead load deflection of both the boom 26 and the mast 16 will remain constant at all rotative locations of the mast 16 and the boom 26 with respect to the base 11. Due to this increased uniformity of deflection of the boom 26, mast 16 and base 11 (which in the last named instance results from the use of the conical reinforcing member the deflection of the boom 26 from horizontal, when rotated, will remain substantially constant throughout all 360 of rotation possible for the boom. Once shims are inserted as described above to obtain correction for dead load deflection, the correction has been properly made for all rotative positions of the boom 26, and no further corrective measures are necessary. This simplicity in correcting for deflection of the crane elements permits ready adaptation of a basic base 11 and mast 16 assembly to a broader range of load capability than has heretofore been possible, as by providing a boom 26 properly sized to support the loads in question and 6 correctly shimming the boom 26 to approximately obtain a true horizontal position.

In order to make clear the unefulness of a basic mast and base assembly of the jib crane of this invention with cranes having varying load capacity ranges, a specific example will now be given of one jib crane construction in accordance with this invention. It is to be understood that the example here given is for purposes of illustration only, and not by way of limitation.

In the design of a jib crane in accordance with this invention, it is the overturning moment, or the bending moment couple forces resulting from the suspension of a weight from the boom as a load, which determines the detailed construction of the radial bearing means and the choice of material dimensions for the mast 16 and the constituent elements of the base 11. For example, in a jib crane constructed as shown in FIGURE 1 which is to have a boom length of 16 feet and to be rated at 10,000-pounds capacity, the mast 16 is formed of steel pipe having an outside diameter of 18 inches and a wall thickness of inch. The base plate is of 1%. inch steel plate and has a -inch diameter. The cage member 14 defining the bore in the base 11 is 4 feet high, and is preferably formed of steel pipe having an outside diameter of 20 inches and a wall thickness of /8 inch. The reinforcing member 15 is rolled from 4 inch steel plate, to have an external diameter at its lower extremity of about 54 inches, and an upper internal diameter of 20 inches with its top just below the upper extremity of the cage member 14 defining the bore. In a jib crane of the capacity mentioned above, the radial bearing means for transmitting the overturning forces desirably are six (6) toroidal rows of loose balls at each of the upper and lower bearing means locations, or a total of 12 rows of balls in all.

A mast and base construction having the particular dimensions described above, for purposes of illustration only, may be used in any jib crane Where the overturning moment is the same as would be obtained in a 10,000- pound capacity crane having a boom length of 16 feet, such as for example a crane having a 20,000-pound capacity and a boom length of 8 feet. Other ranges of useful load capacity for the mast and base assembly should be readily apparent.

In accordance with my invention, a jib crane construction as heretofore described may be employed for a foundation supported and relatively permanently installed crane as illustrated in FIGURE 6 in distinction to a base assembly supported crane (FIGURES 14). A modiied form of jib crane in accordance with this invention, indicated generally at 30 (FIGURE 6), is shown and will now be described in order to make apparent the adaptation of my invention to a foundation supported crane. In the foundation supported crane 30, there is no need to provide a base plate 12 and conical reinforcing member 15. In distinction, a cage member 31, similar to the cage member 14 described above, is provided and set into a foundation block 32, which suitably may be of concrete and cast in situ, as beneath a building or dock floor 33. Preferably, the cage member 31 is cast into the block 32 to project a short distance above the surface of the floor 33, to preclude easy contamination of the enclosed bearing means by floor sweepings and the like. In order to provide a lower horizontally extending thrust bearing surface, it is preferred to provide a closure plate 34 at the lower end of the cage member 31, which may be flame cut from suitable steel plate without machining and welded to the cage member 31 in a manner similar to the attachment of the cage member 14 to the base plate 12 as described heretofore. Other elements of the foundation supported jib crane 31 (including those not shown in FIGURE 6) may be substantially identical to those described above With reference to the base assembly supported jib crane 10, and are identified by primed numerals (Where illustrated).

In accordance with this invention, provision is made for further extending the adaptability of the base and mast construction to still broader ranges of load capability. More particularly, a modified form of jib crane, indicated generally at 46, is shown in FIGURES 7 and 8, where substantially identical elements of the construction are identified by double primed numerals similar to the numerals used to identify elements of the crane shown in FIGURES 1-4.

The modified jib crane 49 of FIGURES 7 and 8 is a base assembly supported crane adapted for use with heavier loads, and increased ranges of load capability, by reinforcing members which increase the rigidity of the mast 16", thereby reducing the deflection of the mast under load. More particularly, where the mast 16" is a section of hollow cylindrical form, such as a pipe, an internal mast reinforcing member 41 is secured within the hollow cylindrical member adjacent the lower end thereof. Inasmuch as the mast 16" rotates Wtih the boom 26", the inner reinforcing member 41 may be and is, in accordance with this invention, positioned to have the axis XX of maximum sectional properties of the member erpendicular to the longitudinal axis Y-Y of the boom 26, along which load forces are imposed, thereby lending increased stiffness to the lower portion of the mast l6 and reducing the defiection of and unit stress in that portion of the mast. The increased stiffness and reduced stress thus provided does not vary as the boom 26" is moved to various rotative locations, as the perpendicular relationship of the axes XX and Y-Y is maintained. Should a section other than a hollow cylindrical section be chosen for use as the mast 16", similar reinforcing members may be added where necessary to increase the sectional properties of the mast 16".

Upwardly of the portion of the mast 16 which is received within the base 11", an external reinforcing member 42, which may be in the form of a generally triangular gusset plate of suitable steel plate, is secured to the exterior of the mast 16", such as by welding. The exterior reinforcing member 42, in similarity to the internal reinforcing member 41, has the axis X'X of maximum sectional properties perpendicular to the longitudinal axis Y-Y of the boom 26", to thereby lend increased stiffness to the upper portion of the mast l6" and reduce the deflection and stress of the mast under load. With the mast 16" reinforced as here described, one basic assembly of a base 11" and a mast 16" may be used with a wide range of load capacity cranes, thereby further increasing the interchangeability of elements between various cranes and simplifying manufacturing procedures.

In summary, it is believed apparent that jib cranes have been illustrated and described which provide the desirable results of reduced expense of manufacture and improved facility in use, through greater interchangeability of parts, more accurate control over the relative positioning of the boom under dead load, and a reduction in the effort required to make use of the crane. This is obtained through a simplified construction using inexpensive components to provide a mast which rotates with the boom and is rotatably received within a base. Thrust and radial hearing means, in the form of loosely disposed uniformly sized balls, are interposed respectively between the lower extremity of the mast and the base to rotatably support the mast against vertically downwardly directed thrust forces and between the mast and base at vertically spaced areas to rotatably support the mast against overturning forces.

In the drawings and specifications there have been set forth preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being defined in the claims.

I claim:

1. A readily rotatable jib crane adaptable to varying load capacities and comprising (a) a base having side and bottom wall means defining a vertically extending bore, said side wall means having a generally vertically extending, unmachined interior surface and said bottom wall means having a horizontally extending substantially planar, unmachined upper surface,

(b) a vertically extending mast having a lower portion thereof rotatably mounted in said bore, said lower portion of said mast having an unmachincd outer surface and a horizontally extending substantially planar, unmachined lower end,

(c) a first plurality of loose ball members randomly disposed between and in direct contact with the unmachined surfaces of said bottom wall means and the lower end of said mast, and

(d) a second plurality of ball members disposed in at least two spaced apart toroidal rows between and in direct contact with said unmachined surfaces of said side Wall means and said lower portion of said mast, said ball means in said toroidal rows being sufficiently loosely arranged for the rolling contact with said unmachined surfaces.

2. A jib crane according to claim 1 wherein said second ball members are disposed in at least one upper toroidal row and at least one lower toroidal row at spaced apart points on the lower portion of said mast, and said ball members are retained at each of said spaced apart points by at least upper and lower annular members secured to said mast.

3. A jib crane according to claim 2 wherein said second ball members are disposed in a plurality of toroidal rows at said upper and lower points on said mast and each row is separated from the immediately adjacent row by a floating annular member disposed between the rows and mounted for vertical movement relative to said mast.

4. A jib crane according to claim 1 wherein said base comprises a base plate, a cylindrical hollow member secured to said base plate and defining said side wall means, the area of said base plate within said hollow member defining said bottom wall means, and a conical reinforcing member secured at the upper extremity thereof to said cylindrical member and diverging downwardly therefrom and being secured at the lower extremity thereof to said base plate.

5. A jib crane according to claim 1 including an elongate boom mounted on the upper end of said mast for rotation therewith, and wherein said mast comprises a hollow cylindrical member, an interior reinforcing member secured within said lower portion of said mast, and an exterior reinforcing member secured to the exterior of said mast upwardly of said lower portion, said reinforcing members having their greatest resistance to bending loads disposed parallel to the longitudinal axis of said boom.

6. A jib crane according to claim 5 wherein said interior reinforcing member comprises an elongate, fiat member extending within said mast at least between said upper and lower rows of ball members and having its longest transverse axis disposed parallel to the longitudinal axis of the boom.

References Cited UNITED STATES PATENTS 610,341 9/1898 Brown 308-230 2,109,304 2/1938 Ormsby 212-- 2,301,105 11/1942 Yost 308230 2,414,573 1/1947 Wagner et al. 2l270 ANDRES H. NIELSEN, Primary Examiner. 

1. A READILY ROTATABLE JIB CRANE ADAPTABLE TO VARYING LOAD CAPACITIES AND COMPRISING (A) A BASE HAVING SIDE AND BOTTOM WALL MEANS DEFINING A VERTICALLY EXTENDING BORE, SAID SIDE WALL MEANS HAVING A GENERALLY VERTICALLY EXTENDING, UNMACHINED INTERIOR SURFACE AND SAID BOTTOM WALL MEANS HAVING A HORIZONTALLY EXTENDING SUBSTANTIALLY PLANAR, UNMACHINED UPPER SURFACE, (B) A VERTICALLY EXTENDING MAST HAVING A LOWER PORTION THEREOF ROTATABLY MOUNTED IN SAID BORE, SAID LOWER PORTION OF SAID MAST HAVING AN UNMACHINED OUTER SURFACE AND A HORIZONTALLY EXTENDING SUBSTANTIALLY PLANAR, UNMACHINED LOWER END, (C) A FIRST PLURALITY OF LOOSE BALL MEMBERS RANDOMLY DISPOSED BETWEEN AND IN DIRECT CONTACT WITH THE UNMACHINED SURFACES OF SAID BOTTOM WALL MEANS AND THE LOWER END OF SAID MAST, AND (D) A SECOND PLURALITY OF BALL MEMBERS DISPOSED IN AT LEAST TWO SPACED APART TORODIAL ROWS BETWEEN AND IN DIRECT CONTACT WITH SAID UNMACHINED SUFACES OF SAID SIDE WALL MEANS AND SAID LOWER PORTION OF SAID MAST, SAID BALL MEANS IN SAID TOROIDAL ROWS BEING SUFFICIENTLY LOOSELY ARRANGED FOR THE ROLLING CONTACT WITH SAID UNMACHINED SURFACES. 